Very finely divided rare earth sulfide colorant compositions

ABSTRACT

Unique colorant compositions, well suited for the pigmentation of a wide variety of coatings and substrates, e.g., cosmetics, plastics, paints and rubbers, comprise (1) at least one crystalline rare earth metal sulfide or sesquisulfide, for example cubic cerium sesquisulfide Ce 2  S 3 , and (2) a dopant amount of at least one alkali metal, and wherein the at least one rare earth metal sulfide or sesquisulfide comprises whole (unground) monocrystalline grains thereof having a mean particle size of at most 1.5 μm.

This application is a continuation of application Ser. No. 08/436,725,filed May 8, 1995 now abandoned.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to very finely divided colorantcompositions based on rare earth metal sulfides and comprising at leastone alkali metal, to a process for the preparation thereof and to theuse of same for the improved pigmentation of a wide variety of materialsand substrates.

2. Description of the Prior Art

Inorganic colorant pigments are today widely used in many industries,especially in those of paints, plastics and ceramics. For theseapplications, such properties as, inter alia, thermal and/or chemicalstability, dispersibility (ability of the material to disperse properlyin a given medium), intrinsic color, coloring capacity and opacifyingpower constitute particularly significant criteria to be considered inthe selection of a suitable pigment.

Unfortunately, the problem remains that the majority of the inorganicpigments which are suitable for applications such as those indicatedabove and which are currently effectively used on an industrial scalegenerally comprise metals (cadmium, lead, chromium and cobaltespecially), the use of which is becoming increasingly strictlycontrolled, indeed banned, by law in many countries because of theirallegedly very high toxicity. More particularly exemplary are the redpigments based on cadmium selenide and/or cadmium sulfoselenide and forwhich substitutes based on the rare earth metal sulfides are alreadyknown to this art. Compositions based on rare earth metal sesquisulfidesand alkali metal elements have thus been described, in EP-A-545,746.These compositions, which are obtained via a process comprising heatinga mixture based on a rare earth metal compound, an alkali metal elementand sulfur, have proven to be particularly advantageous substitutes.

However, need continues to exist for substitute materials havingpigment-grade properties which are yet further improved and which can beprepared via an industrially even simpler process.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofimproved colorant compositions which avoid or conspicuously amelioratethe above disadvantages and drawbacks to date characterizing the stateof this art.

Briefly, the present invention features novel colorant compositionsbased on a rare earth metal sulfide and which comprise at least onealkali metal element, said metal sulfide comprising wholemonocrystalline grains, the grains having a mean particle size of atmost 1.5 μm.

This invention also features a process for the preparation of such novelcolorant compositions based on a rare earth metal sulfide and at leastone alkali metal, comprising contacting at least one rare earth metalcarbonate or hydroxycarbonate with at least one compound of an alkalimetal element and then heating the resulting admixture in the presenceof at least one gas which comprises hydrogen sulfide or carbondisulfide.

DETAILED DESCRIPTION OF BEST MODE AND PREFERRED EMBODIMENTS OF THEINVENTION

More particularly according to the present invention, the subjectcolorants have an especially fine particle size, notably less than 2 μm,and very high coloration properties. The process for the preparationthereof is simple to carry out and, additionally, can be operated atrelatively low temperatures, for example from about 500° C.

The colorant compositions of the invention will now be more fullydescribed.

The subject colorant compositions are based essentially on at least onerare earth metal sulfide. Such at least one rare earth metal sulfide isvery advantageously a sesquisulfide.

By "rare earth metal" are intended the elements belonging to thelanthanide group of the Periodic Table having an atomic number of from57 to 71, as well as yttrium which has an atomic number of 39. By"Periodic Table" is intended that published in the Supplement to theBulletin de la Societe Chimique de France, No. 1 (January 1966). It willalso be appreciated that the sulfide or the sesquisulfide of thecompositions of the invention can comprise a number of rare earth metalsand, thus, in the description that follows reference to a rare earthmetal also applies to the case in which a number of rare earth metalsare present.

This invention is particularly applicable to the sesquisulfides in whichthe rare earth metal is cerium or lanthanum.

The sulfide or the sesquisulfide of the compositions of the inventionadditionally comprises at least one alkali metal element (dopingelement, or dopant). This alkali metal element is especially selectedfrom among lithium, sodium or potassium. Sodium is particularlypreferred. Of course, the sulfide or sesquisulfide of the compositionsof the invention can comprise a number of alkali metal elements and,again, reference to an alkali metal also applies to the case in which anumber of alkali metals are present.

In a preferred embodiment of the invention, this alkali metal element isat least partially confined or enclosed within the crystalline latticeof the sulfide or sesquisulfide. In another embodiment, the alkali metalelement is essentially or completely enclosed within the crystallinelattice.

The sesquisulfide of the compositions of the invention is, especially, acubic crystallographic structure of Th₃ P₄ type, which has gaps or voidsin the cation lattice; this lacunary structure can be symbolized byaccording the sesquisulfides the formula M₁₀.66 !₁.33 S₁₆ (in thisregard, see, in particular, W. H. Zachariasen, "Crystal Chemical Studiesof the 5f-Series of Elements. The Ce₂ S₃ Ce₃ S₄ Type of Structure", ActaCryst., 2, 57 (1949)).

According to the invention, one or more alkali metal elements can beintroduced into these cationic gaps, optionally up to saturation of thelatter. The presence of this element within the sulfide or sesquisulfidecan be demonstrated by simple chemical analysis. Moreover, X-raydiffraction analyses evidences that the Th₂ P₄ crystalline phase of thesesquisulfide is retained with, in certain instances, modification ofthe unit cell parameters to a greater or lesser extent, depending bothon the nature and on the amount of the doping element introduced.

Generally, the amount of alkali metal element is at most 50% of themolar amount of the rare earth metal in the sulfide or in thesesquisulfide.

In another preferred embodiment of the invention, the molar amount ofalkali metal is at least equal to 0.1% and advantageously ranges from 5%to 50% and more preferably from 5% to 20% of the molar amount of rareearth metal.

An essential characteristic of the compositions of the invention is thatthey are based on a sulfide comprising whole monocrystalline grains, thegrains having a mean particle size of at most 1.5 microns and preferablyat most 1 micron. By "whole grain" is intended a grain which has notbeen broken or crushed. Grains can indeed be broken or crushed duringmilling. Scanning electron microscopy photographs of the product of theinvention demonstrate that the grains of which it is comprised have notbeen crushed. It will also be appreciated that the sulfide or thesesquisulfide of the compositions of the invention can bedeagglomerated, namely, if it is not provided directly in the form ofwhole monocrystalline grains, it can be provided in the form ofagglomerates of particles which can comprise agglomerated and/orslightly sintered grains which can be converted, by deagglomerationunder mild conditions, into the whole monocrystalline grains. Thecombination of the nature of the grain (whole particulates) and itssmall particle size is likely the explanation for the very goodpigmentation properties of the composition of the invention.

The particulates of the invention, preferably have a mean particle sizewhich is generally less than 2 μm and typically ranges from 0.7 to 1.5μm. After deagglomeration under mild conditions, the mean particle sizeis at most 1.5 μm and advantageously ranges from 0.3 to 0.8 μm. The sizeof the particles is measured by Cilas particle size measurement.

The compositions based on sulfides or on sesquisulfides of the inventionpresent a very wide range of colors depending especially on the rareearth metal and on the alkali metal element comprising the sulfides orsesquisulfides which indicates that their chromatic coordinates can varyover a very wide range. Exemplary thereof are the following:

(a) cerium sulfides have a color varying from brown to red depending onthe preparation conditions, in particular the calcination temperature.They are brown or blood-red depending on whether the β-orthorhombic Ce₂S₃ phase or γ-cubic Ce₂ S₃ phase is present;

(b) with lanthanum, yellow products are obtained having a cubic La₂ S₃structure;

(c) a green coloration is obtained with neodymium and a yellow/greencoloration with praseodymium. The products then respectively have thecubic Nd₂ S₃ structure and the cubic Pr₂ S₃ structure;

(d) a brown/yellow product is obtained with dysprosium of cubic Dy₂ S₃structure;

(e) products having various shades of brown can also be obtained: ochrewith terbium of cubic Tb₂ S₃ structure, brown with erbium of monoclinicEr₂ S₃ structure and dark beige with yttrium of monoclinic Y₂ S₃structure;

(f) lastly, other examples of colors which can be obtained are:grey/brown with samarium of cubic Sm₂ S₃ structure, green/brown withgadolinium of γ-cubic Gd₂ S₃ structure or gold/green with thulium ofmonoclinic Tm₂ S₃ structure.

The process for the preparation of the compositions of the inventionwill now be more fully described.

A first characteristic of the subject process is nature of the startingmaterials. The rare earth metal is introduced in the form of a carbonateor of a hydroxycarbonate.

It is advantageous to use a carbonate or hydroxycarbonate having a fineparticle size and, especially, having a mean particle size of at most 1μm.

As regards the alkali metal element, it can be introduced in variousforms. Exemplary thereof are the salts of these elements. However, it isadvantageous to employ an alkali metal carbonate.

Preferably, a powder based on a homogeneous mixture of the rare earthmetal carbonate or hydroxycarbonate with the compound of an alkali metalelement is constituted.

In one embodiment of the invention, a rare earth metal carbonate orhydroxycarbonate is employed that has been impregnated beforehand withan alkali metal element. In this event, an aqueous solution of an alkalimetal salt or hydroxide is formed and the rare earth metal carbonate orhydroxycarbonate is impregnated with this solution and then dried,according to any technique which limits the formation of oxide, forexample in an oven, spray-drying, and the like.

Another characteristic of the process of the invention is the nature ofthe sulfurizing gas. This gas can be hydrogen sulfide or carbondisulfide. In a preferred embodiment of the invention, a mixture ofthese two gases is used. The use of such a mixture promotes theproduction of physically pure products and limits the deposition ofcarbon onto the product obtained. The sulfurizing gas or gas mixture canbe used with an inert gas, such as argon or nitrogen.

The heating is carried out at a temperature which can be as low as 500°C. Indeed, the formation of the desired products has been observed atthis temperature. This is a significant advantage with respect to knownprocesses which require high temperatures, generally of at least 900° C.Typically, the process of the invention is carried out at a temperatureranging from 500° to 900° C., the higher temperatures promoting theproduction of phasically pure products.

The heating time corresponds to the time required to produce the desiredsulfide or sesquisulfide and this time becomes shorter as thetemperature becomes higher. For example, this time can range fromapproximately two hours for a temperature of 500° C. to approximatelyfifteen minutes for a temperature of 800° C.

The reaction is generally carried out employing a hydrogen sulfideand/or carbon disulfide partial pressure ranging from 0.1 to 1×10⁵ Pa.

Lastly, the process can be carried out in an open reactor.

The product obtained on completion of the heating usually has a meanparticle size of less than 2 μm and more particularly less than 1.5 μm.However, if it is desired to obtain a finer particle size, the productcan be deagglomerated. As indicated above, a deagglomeration under mildconditions, for example using air-jet-type milling, is sufficient toobtain a mean particle size which can be less than 1.5 μm and, forexample, at most 1 μm and advantageously ranging from 0.3 to 0.8 μm.

The present invention also features colored pigments comprising acomposition based on at least one sulfide or sesquisulfide as describedabove, or produced via the subject process.

The compositions based on sulfides or on sesquisulfides or the pigmentsaccording to the invention have a very good coloration power and a verygood covering or coating capacity. For this reason, they are perfectlysuited for the coloration of a wide variety of substrates and materials,such as plastics, paints and others.

Thus, they are well suited for the coloration of plastics which can beof thermoplastic or thermosetting type.

Exemplary thermoplastic resins suited for coloration according to theinvention include poly(vinyl chloride), poly(vinyl alcohol),polystyrene, styrene/butadiene, styrene/acrylonitrile oracrylonitrile/butadiene/styrene (A.B.S.) copolymers, acrylic polymers,especially poly(methyl methacrylate), polyolefins such as polyethylene,polypropylene, polybutene or polymethylpentene, cellulose derivativessuch as, for example, cellulose acetate, cellulose acetobutyrate orethyl cellulose, or polyamides, for example, polyamide-6,6.

Exemplary thermosetting resins for which the compositions or pigmentsaccording to the invention are also suitable include, for example,phenoplasts, aminoplasts, especially urea/formaldehyde ormelamine/formaldehyde copolymers, epoxy resins and thermosettingpolyesters.

The compositions or pigments of the invention can also be used for thecoloration of special polymers such as fluorinated polymers, inparticular polytetrafluoroethylene (P.T.F.E.), polycarbonates, siliconeelastomers or polyimides.

In this specific application for the coloration of plastics, thecompositions or pigments of the invention can be used directly in theform of powders. It is also possible, preferably, to employ the subjectcompositions in a predispersed form, for example a premix with afraction of the resin, or in the form of a pasty concentrate, or of aliquid, permitting the compositions to be introduced at any stage in themanufacture of the resin.

Thus, the compositions or pigments according to the invention can beincorporated into plastics such as those indicated above in a proportionby weight generally ranging either from 0.01% to 5% (relative to thefinal product) or from 40% to 70% in the case of a concentrate.

The compositions or pigments of the invention can also be used in thefield of paints and varnishes and, more particularly, in the followingresins: alkyd resins, the most typical of which being referred to asglycerophthalic; resins modified with long or short chain oils; acrylicresins prepared from esters of acrylic (methyl or ethyl) and methacrylicacid optionally copolymerized with ethyl, 2-ethylhexyl or butylacrylate; vinyl resins such as, for example, poly(vinyl acetate),poly(vinyl chloride), poly(vinyl butyral), poly(vinyl formal) and vinylchloride and vinyl acetate or vinylidene chloride copolymers;aminoplastic or phenolic resins, most often modified; polyester resins;polyurethane resins; epoxy resins; or silicone resins.

Generally, the compositions or pigments are employed in a proportion of5% to 30% by weight of the paint and of 0.1% to 5% by weight of thevarnish.

Lastly, the compositions or pigments of this invention are also suitablefor applications in the rubber industry, especially in floor coverings,in the paper and printing inks industry, in the field of cosmetics andin many other applications such as, for example, leather finishing andlaminated coatings for kitchens and other work surfaces, or ceramics.

More particularly in respect of the cosmetics field, the compositions orpigments of the invention are useful in nail varnishes and in makeupproducts such as lipsticks, dry makeups, greasy makeups or foundationcreams.

They can thus be formulated into nail varnishes and polishes whichgenerally contain:

(a) a film-forming agent based on nitrocellulose,

(b) a resin, natural dammar resin or synthetic resin offormaldehyde/sulfamide type, polystyrene resin, polyvinyl resin, and thelike.

(c) a plasticizer, for example diethyl phthalate, dibutyl phthalate,dioctyl phthalate, tricresyl phosphate, n-butyl stearate, resorcinoldiacetate or mixture thereof,

(d) a solvent such as ethyl, isopropyl, butyl or isobutyl alcohol, ethylacetate, butyl acetate or, typically, a mixture of these solvents,

(e) a diluent, especially toluene or xylene,

(f) optionally, other additives and adjuvants, fragrances orpearlessence compounds (mica flakes coated with bismuth oxychloride ortitanium dioxide).

An exemplary such composition is given below:

(i) from 10% to 15% by weight of nitrocellulose,

(ii) from 10% to 15% by weight of resin,

(iii) from 3% to 5% by weight of plasticizer(s),

(iv) from 3% to 5% by weight of pigment(s),

(v) q.s. for 100% by weight of solvent(s).

Generally, the compositions or pigments are milled in a plastic masscomprising nitrocellulose and plasticizer(s), which is then dissolved inthe solvent(s).

Another application of the compositions or pigments of the invention isfor formulation into lipsticks.

The compositions or pigments are most typically incorporated in aproportion of a concentration by weight of 5% to 15% expressed withrespect to the total weight of the formulation which contains:

(a) an excipient formed from a mixture of various materials to providefor consistency: beeswax, carnauba wax, ozocerites, paraffin, syntheticwaxes or mixture thereof and from a soft excipient which permitsadjusting the consistency, such as cocoa butter, petroleum jelly,hydrogenated white oils for example, or palm, groundnut or castor oil,

(b) various additives and adjuvants, especially a fragrance or flavorand isopropyl myristate or isopropyl palmitate which providesslipperiness,

(c) an intermediate solvent for suspending the pigment in the lipophilicphase, which can be castor oil or a glycol, such as polyethylene glycol400, or fatty acid esters: propylene glycol monoricinoleate, isopropylmyristate, isopropyl palmitate or butyl stearate.

The eye shadows and blushes can be provided in the form of dry makeupproducts or greasy makeup products. The content of the subjectcompositions or pigments in such makeups can vary over wide limits, forexample from 5% to 20%.

The dry makeup products are powders (talc, magnesium carbonate or zincstearate) which are laden with pigments and agglomerated either withmethyl cellulose or with stearates.

The following is an exemplary composition for an eye shadow:

    ______________________________________                                        (i)      aluminum magnesium                                                                           7% by weight                                                   silicate (Veegum F):                                                 (ii)     talc:         50% by weight                                          (iii)    zinc oxide:    4% by weight                                          (iv)     zinc stearate:                                                                              11% by weight                                          (v)      kaolin:       10% by weight                                          (vi)     pigment:      18% by weight                                          ______________________________________                                    

The compositions or pigments of the invention can also be incorporatedin foundation cream formulations.

The foundation creams are provided in the form of an emulsion,characteristically of the oil-in-water type.

The lipophilic phase typically comprises:

(a) an oily component such as liquid paraffin, esters of fatty acids andof optionally fatty alcohols, for example oleyl oleate, decyl oleate,octyl stearate, di-n-butyl adipate, isopropyl myristate, isopropylpalmitate, isopropyl stearate or the esters of capric and caprylic acidswith saturated fatty alcohols having from 12 to 18 carbon atoms, asilicone oil or mixture thereof,

(b) an emulsifying agent of anionic and/or nonionic type and,especially, the salts of fatty acids, sodium, potassium or ammoniumstearate or sodium palmitate: the esters of sorbitan and of fatty acidssuch as, for example, lauric acid, palmitic acid or stearic acid; thepolyoxyethylenated esters of sorbitan and of fatty acids containing from4 to 20 mol of ethylene oxide per mole of ester: the polyoxyethylenatedfatty alcohols containing from 2 to 23 mol of ethylene oxide per mole ofalcohol, said alcohol especially being lauryl alcohol, cetyl alcohol,stearyl alcohol or oleyl alcohol; glyceryl mono- and distearate orglyceryl mono- and dioleate; polyoxyethylenated fatty acids and inparticular polyoxyethylenated stearate containing from 18 to 100 mol ofethylene oxide per mole of acid,

(c) an agent for adjusting the consistency of the product, whichadvantageously is a fatty alcohol or a fatty acid and more particularlycetyl alcohol, stearyl alcohol or stearic acid.

With respect to the hydrophilic phase, it comprises water, preferablydistilled, and various additives and adjuvants, especially:

(a) a humectant which can be, for example, propylene glycol, glycerol orsorbitol,

(b) a preservative and more particularly o-phenylphenol and thefollowing acids, their salts (Na, K or NH₄) or their esters having from1 to 4 carbon atoms: benzoic acid, salicylic acid, sorbic acid orp-hydroxybenzoic acid,

(c) a stabilizing agent, especially cellulose derivatives includingcarboxymethyl cellulose and xanthan gum.

An exemplary formulation for a foundation cream is as follows:

    ______________________________________                                        (A) Lipophilic phase:                                                         (i)    liquid paraffin:   15% by weight                                       (ii)   glyceryl mono- and distearate:                                                                   4% by weight                                        (iii)  cetyl alcohol:     1% by weight                                        (B) Hydrophilic phase:                                                        (i)    distilled water q.s. for:                                                                        100% by weight                                      (ii)   propylene glycol:  3% by weight                                        (iii)  methyl para-hydroxybenzoate:                                                                     0.05% by weight                                     (iv)   propyl para-hydroxybenzoate:                                                                     0.1% by weight                                      (v)    colorant pigment:  1 to 10% by weight                                  (vi)   titanium dioxide:  3% by weight                                        ______________________________________                                    

The preparation of the foundation cream formulations is carried out byfirst dispersing the pigment in the lipophilic phase, maintained at atemperature of about 60°-80° C. and then adding the hydrophilic phase,maintained at a temperature within the above range, with stirring andslowly to the lipophilic phase.

In the foregoing description, the reported formulations for cosmeticscomprising the compositions or pigments of the invention are exemplaryonly.

The present invention also features the compositions, substrates andshaped articles comprising the subject colorants. Such compositions,substrates and shaped articles include plastics, paints, varnishes,rubber, ceramics, glazings, papers, inks, cosmetics, dyes, laminatedcoatings, and the like.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

In said examples to follow, the chromatic coordinates L*, a* and b* arereported in the CIE system 1976 (L*, a*, b*) as defined by theInternational Lighting Commission and catalogued in the Compilation ofFrench Standards (AFNOR), calorimetric color No. X08-12 (1983). They aredetermined using a calorimeter marketed by Pacific Scientific. Theilluminant was D65. The observation surface was a circular pellet havinga surface area of 12.5 cm². The observation conditions correspond toviewing at an aperture angle of 10°. The specular component was excludedfrom the measurement reported.

L* is a measurement of the reflectance (light/dark shading) and thusranges from 100 (white) to 0 (black).

a* and b* are the values of the color trends:

a* positive=red

a* negative=green

b* positive=yellow

b* negative=blue

L* thus represents the variation from black to white, a* the variationfrom green to red and b* the variation from yellow to blue.

EXAMPLE 1

This example relates to the preparation of a cubic cerium sesquisulfidecomprising sodium values.

6 g of cerium hydroxycarbonate, having a Cilas particle size less than 1μm, and 0.22 g of anhydrous sodium carbonate were introduced into amortar. The Na/Ce molar ratio was then 0.2.

The entire mixture was then ground to provide a homogeneous mixture.This mixture was then heated to 500° C. for 2 hours, while continuouslysweeping the reaction zone with a gas mixture containing argon, hydrogensulfide and carbon disulfide (50%, 20% and 30% respectively by volume).

The product thus obtained was principally cubic cerium sesquisulfide, asdetermined by X-ray diffraction analysis. Its particle size was lessthan 1 μm.

EXAMPLE 2

A cubic cerium sesquisulfide doped with lithium was prepared bycalcining a mixture of cerium hydroxycarbonate, of the same type as thatin Example 1, and lithium carbonate at a constant temperature of 800° C.for 90 minutes under an atmosphere containing carbon disulfide, in theproportion of a partial pressure of 0.3×10⁵ Pa, and hydrogen sulfide, inthe proportion of 0.2×10⁵ Pa, in argon. The Li/Ce molar ratio in themixture was 0.10.

The product obtained was a red pigment whose color coordinates andparticle size were the following:

L*=45.1

a*=46.1

b*=34.1

D₅₀ =1.5 μm

After simple deagglomeration, the product obtained was in the form ofwhole monocrystalline grains having a particle size of less than onemicron. The D₅₀ of the product was 0.65 μm.

EXAMPLE 3

In contradistinction to the above examples, the alkali metal wasuniformly distributed in the rare earth metal precursor by impregnationof said precursor with the alkali metal salt (sodium carbonate) insolution. The alkali metal was here present in an Na/Ce ratio of 0.15.After treatment with a sulfurizing mixture containing 30% (by volume) ofCS₂, 20% of hydrogen sulfide and the remainder as argon, at a constanttemperature of 800° C. for 30 minutes, the sesquisulfide wascrystallized in the Th₃ P₄ structure.

Its color characteristics were the following:

L*=55.9

a*=50.3

b*=43.3

D₅₀ =1 μm

After deagglomeration, the product obtained had a D₅₀ of 0.65 μm andwhole monocrystalline grains having a particle size of less than onemicron.

EXAMPLE 4

This example describes the synthesis of a cerium sulfide having a verysmall particle size.

Under a partial pressure of 0.3×10⁵ Pa of CS₂, 0.2×10⁵ Pa of H₂ S andthe remainder as argon, a cerium carbonate having a Cilas particle sizeof 0.7 μm was converted, in the presence of sodium carbonate in an Na/Ceratio of 0.15, into cubic cerium sulfide, at a constant temperature of800° C. for 30 min. It had the following color characteristics andparticle size:

L*=55.6

a*=46.2

b*=45.1

D₅₀ =0.9 μm

After simple deagglomeration, the mean diameter was 0.55 μm. The productwas in the form of whole monocrystalline grains having a particle sizeof less than one micron.

EXAMPLE 5

In contradistinction to the above examples, a mixture containing onlycarbon disulfide as the sulfurizing agent, diluted to 30% in argon, wasemployed. 6 g of cerium hydroxycarbonate were mixed with 0.29 g ofsodium carbonate and the mixture was heated to 800° C. for 5 h. Aproduct was obtained which was divided into two fractions according to acolor criterion. The X-ray analyses evidenced, indeed, a pure cubicphase for a red fraction and a cubic phase, as well as a brownoxysulfide phase, for the second fraction.

The color coordinates of the reddest fraction were the following:

L*=49.8

a*=48.8

b*=40.2

D₅₀ =2.0 μm

After deagglomeration, the mean diameter of the particulates was 1.45μm.

Sulfurization in the presence of carbon disulfide and hydrogen sulfide(30% and 20% by volume, the remainder as argon) of the same ceriumhydroxycarbonate produced a single cubic cerium sesquisulfide phasewhose chromatic coordinates were the following:

L*=50.7

a*=51.5

b*=42.1

The product was in the form of whole monocrystalline grains having aparticle size of less than one micron.

EXAMPLE 6

A cerium hydroxycarbonate having a particle size of less than 1 μm wasmixed with a sodium carbonate in the proportion of an Na/Ce ratio=0.15.This mixture, heated to a constant temperature of 750° C. for 2 h, undera sulfurizing atmosphere of CS₂, H₂ S and argon (30%, 20% and 50%,respectively, by volume), produced a cerium sulfide of Th₃ P₄ type whosecolor coordinates were the following:

L*=52.2

a*=50.4

b*=46.7

D₅₀ =1.4 μm

After deagglomeration, the mean diameter was 0.8 μm. The product was inthe form of whole monocrystalline grains having a particle size of lessthan one micron.

EXAMPLE 7

In this example, the synthesis was carried out of a pigment, employing avery short thermal cycle: 9 g of a cerium hydroxycarbonate having aparticle size of less than 1 μm, mixed with 0.31 g of sodium carbonate,were heated at a temperature of 800° C. for 15 min in the presence of amixture of argon, hydrogen sulfide and carbon disulfide at,respectively, 50%, 20% and 30% by volume. The chromatic coordinates ofthe final product were the following:

L*=51.3

a*=50

b*=42.6

D₅₀ =1.5 μm

The product was in the form of whole monocrystalline grains having aparticle size of less than one micron.

EXAMPLE 8

This example relates to the preparation of a mixed cerium lanthanumsulfide.

A cerium lanthanum hydroxycarbonate having a Ce/La molar ratio of 3 andhaving a particle size of less than one micron was sulfurized at aconstant temperature of 800° C. for 30 minutes in the presence of sodiumcarbonate in a proportion of 20 molar % with respect to the rare earthmetals. The sulfurizing gas was a mixture of carbon disulfide, hydrogensulfide and argon in the respective proportions of 30%, 20% and 50% byvolume. The color and particle size characteristics thereof were thefollowing:

L*=56.3

a*=50.7

b*=48.9

D₅₀ =1.7 μm

After deagglomeration, the mixed sesquisulfide had a mean size of 0.85μm. The product was in the form of whole monocrystalline grains having aparticle size of less than one micron.

EXAMPLE 9

A praseodymium sulfide was prepared by reacting a gas mixture containing30% by volume of carbon disulfide and 15% of hydrogen sulfide, theremainder to 100 being contributed by argon, with a mixture ofpraseodymium carbonate and sodium carbonate (Na/P ratio of 0.2). Thetemperature of the synthesis was 800° C. and the constant temperatureperiod was 5 hours.

The sesquisulfide obtained had a particle size of less than one micronand its chromatic coordinates were the following:

L*=86.9

a*=-15.2

b*=57.8

The product was in the form of whole monocrystalline grains having aparticle size of less than one micron.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. A colorant composition of matter which comprises(1) at least one crystalline rare earth metal sulfide or sesquisulfideand (2) a dopant amount of at least one alkali metal, said at least onerare earth metal sulfide or sesquisulfide comprising grains thereofhaving a mean particle size of at most 1.5 μm wherein said grains arewhole monocrystaline grains.
 2. The colorant composition as defined byclaim 1, wherein at least a fraction of said at least one alkali metalbeing included within a crystal lattice of said at least one rare earthmetal sulfide or sesquisulfide.
 3. The colorant composition as definedby claim 2, said at least one alkali metal being included in cation gapswithin the crystal lattice of said at least one rare earth metal sulfideor sesquisulfide.
 4. The colorant composition as defined by claim 1,having a molar amount of said at least one alkali metal of at most 50%of the molar amount of the at least one rare earth metal.
 5. Thecolorant composition as defined by claim 4, the molar amount of said atleast one alkali metal ranging from 5% to 50% of the molar amount of theat least one rare earth metal.
 6. The colorant composition as defined byclaim 1, wherein said at least one alkali metal comprises sodium.
 7. Thecolorant composition as defined by claim 1, wherein said at least onerare earth metal sulfide or sesquisulfide has a Th₃ P₄ crystallographicstructure.
 8. The colorant composition as defined by claim 1, whereinsaid at least one rare earth metal sulfide or sesquisulfide comprisesγ-cubic cerium sesquisulfide Ce₂ S₃.
 9. The colorant composition asdefined by claim 1, wherein said at least one rare earth metal sulfideor sesquisulfide comprises cubic lanthanum sesquisulfide La₂ S₃.
 10. Aprocess for the preparation of the colorant composition as defined byclaim 1, comprising mixing a carbonate or hydroxycarbonate of at leastone rare earth metal with at least one alkali metal compound to form amixture, heating the mixture thus produced in the presence of at leastone gas which comprises hydrogen sulfide, carbon disulfide, or mixturethereof to form a product, and, optionally, deagglomerating the product.11. The process as defined by claim 10, wherein said at least one gascomprises a mixture of hydrogen sulfide and carbon disulfide.
 12. Theprocess as defined by claim 10, said at least one alkali metal compoundcomprising an alkali metal carbonate.
 13. The process as defined byclaim 10, said carbonate or hydroxycarbonate of at least one rare earthmetal having been impregnated with at least one alkali metal.
 14. Theprocess as defined by claim 10, carried out in an open reactor.
 15. Theprocess as defined by claim 10, wherein the hydrogen sulfide and/orcarbon disulfide has a partial pressure of from 0.1 to 1×10⁵ Pa.
 16. Acolorant composition of matter which comprises (1) at least onecrystalline rare earth metal sulfide or sesquisulfide and (2) a dopantamount of at least one alkali metal, said at least one rare earth metalsulfide or sesquisulfide being in the form of agglomerates whichcomprise agglomerated grains having a mean particle size of at most 1.5μm wherein said grains are whole monocrystalline grains.
 17. Thecolorant composition of matter as defined by claim 1, wherein saidgrains have a mean particle size of between 0.3 and 0.8 μm.